1. Field
Embodiments of the present invention relate to an electronic parking brake, and more particularly, to a decelerator to which a belt gear for an electronic parking brake for a vehicle is applied.
2. Description of the Related Art
In general, parking brakes are devices that stop vehicles such that the vehicles are not moved when the vehicles are parked, and serve to hold wheels of the vehicle such that the wheels are not rotated.
Electronic parking brake (EPB) systems for electronically controlling driving of a parking brake are recently used. These EPB systems are mounted on general disk brakes and perform a function of the parking brake. Here, examples of the EPB systems include cable puller type EPB systems, motor-on-caliper type EPB systems, and hydraulic parking brake type EPB systems.
Korean Unexamined Patent Application Publication No. 10-2013-0038432 discloses a motor-on-caliper type electronic parking brake. Referring to the disclosed literature, the electronic parking brake includes a disk that rotates together with wheels of a vehicle, a carrier having a pair of pad plates located at both sides of the disk to press the disk, a caliper housing that is slidably installed in the carrier and has a piston installed to advance and retreat to press the pair of pad plates, the piston being built in the caliper housing, a motor that generates a driving force, a decelerator that amplifies a driving force generated by a motor, a gear assembly that transfers the driving force of the motor to the decelerator, and a conversion unit that transfers a rotational force of the motor to the piston from the decelerator.
The pair of pad plates may be classified into an inner pad plate adjacent to the piston and an outer pad plate disposed at an opposite side to the inner pad plate.
A cylinder is disposed on one side of the caliper housing, and the piston that presses the inner pad plate toward the disk is installed in the cylinder. A finger part that is bent downward on the other side of the caliper housing is connected integrally with the cylinder, and is capable of pressing the outer pad plate toward the disk together with sliding movement of the caliper housing.
The carrier is fixed to a body of the vehicle, prevents escape of the pair of pad plates, and is disposed to guide the pair of pad plates toward and away from the disk.
The piston performs a rectilinear reciprocal motion through driving of the motor when a brake operation is performed and presses the inner pad plate toward the disk. A driving force of the motor is transferred to the decelerator using the gear assembly and is transferred to the piston using the conversion unit in a state in which the driving force is amplified by the decelerator.
The conversion unit serves to press the piston toward the inner pad plate, as described above. The conversion unit includes a spindle member which is screw-coupled to a rotation shaft of the carrier of the decelerator that will be described later and which receives a rotational force of the motor, and a nut member that is screw-coupled to the spindle member and presses the piston. In this case, a bearing is installed in the cylinder to support the spindle member.
The gear assembly includes a driving gear installed at a shaft of the motor, a driven gear connected to the decelerator, and a pinion idle gear that connects the driving gear and the driven gear. That is, a rotational force generated when the shaft of the motor is rotated is transferred to the driven gear through the pinion idle gear meshed between the driving gear and the driven gear.
Meanwhile, the gear assembly is configured to transfer the rotational force of the motor through a plurality of gears or to transfer the rotational force through a belt pulley, as mentioned above.
The decelerator is configured to have a two-stage planetary gear shape. That is, the decelerator includes a first decelerator part, a second decelerator part, and internal gears.
The first decelerator part includes a first solar gear installed on a central shaft of the driven gear, a plurality of first planetary gears disposed around the first solar gear to be meshed with the first solar gear, and a first carrier connected to shafts of the plurality of first planetary gears.
The second decelerator part has the same structure as that of the first decelerator part. That is, the second decelerator part includes a second solar gear installed on a rotation shaft of a first carrier, a plurality of second planetary gears disposed around the second solar gear to be meshed with the second solar gear, and a second carrier connected to shafts of the plurality of second planetary gears, wherein a rotation shaft of the second carrier is connected to the conversion unit. In this case, the first and second planetary gears are meshed with the internal gears fixed to the outside.
That is, in the above-described electronic disk brake, the rotational force is transferred to the decelerator using the gear assembly by an operation of the motor. Thus, the second planetary gears meshed with the fixed internal gears revolve when the first solar gear is rotated, and the revolution of the second planetary gears is transferred to the second decelerator part through the first carrier. Also, the second decelerator part transfers the rotational force to the spindle member by performing the same operation as that of the first decelerator part so that deceleration rotation of the spindle member can be performed. When the spindle member is rotated, axial movement of the nut member is performed, and the nut member presses the piston so that the brake operation can be performed.
However, the above-described electronic parking brake has a structure in which the driving force of the motor is primarily decelerated using the gear assembly or the belt pulley structure, is finally decelerated using the decelerator having the two-stage planetary gear shape, and is converted into a rectilinear force using the conversion unit to generate a brake force, i.e., a U-shaped power transmission structure so that the sizes of the cylinder, the carrier and the power transmission unit (motor, gear assembly, and decelerator) are increased when the disk brake is mounted on the vehicle and thus, there is a problem of limitation in that the disk brake needs to be installed in only a midsize or more vehicle.
Also, alignment of the entire brake system is displaced by an axial reaction of the motor when the brake operation is performed by gears configured in multi-stages. Thus, noise occurs in the motor or decelerator, and the life span of the motor is reduced.
Thus, various researches and developments on improvements in utility of an installation space for the electronic parking brake that automatically operates the brake using the motor, or a reduction in operating noise have been carried out.